In reference to FIG. 1, in the field of electron beam welding, one wishes to control the beam 12 so that the focal point 14 coincides with the surface 16 of the workpiece 18 to be welded. When focal point 14 is just below surface 16, optimal circumstances occur: the cross-sectional size of beam 12 is at a minimum, thereby causing the current density of the beam 12 at that location to be a maximum and the depth to which the weld extends down into workpiece 18 ("weld depth") to be a maximum. In general, maximum depth and a narrow fusion zone for the selected power input are desired. Where the focal point 20 is significantly below the surface 16 of workpiece 18 or focal points 22 or 24 are above the surface 16, the current density at the surface 14 is not at a maximum because the cross-sectional size of beam 12 at surface 16 is not at a minimum. Consequently, weld depth, and therefore a narrow width fusion zone, are not maximized.
Comparisons of results obtained from different operators and electric welding machines indicate variations of .+-.20% to .+-.40% variation in weld depth for the same nominal power input. "Prediction of Electron Beam Depth of Penetration," Giedt, W. H., and Tallerico, L. N., Welding Journal, Research Supplement, pp. 299-s to 305-s, December 1988. Beam focus location has been shown to have a major influence on penetration. Hence, a basic problem in the art is to focus the beam 12 so that its focal point 14 is just below the surface 16 of workpiece 18.
A number of factors affect the beam focal point: beam current; beam voltage; filament current; focus coil current; travel speed; distance from the electron gun to the workpiece; chamber vacuum level; etc. Although an error or deviation in any machine setting will have some influence on penetration, the most important factor in determining beam focus location has been found to be the focus coil current. In other words, to focus the beam, one adjusts the current passing through the focus coil, or magnetic lens, 6. The focus coil, or magnetic lens, 6 is below the electron gun anode as illustrated in FIG. 6.
A visual and manual version of this approach requires an operator of an electron-beam-welder to position a piece of metal having a high melting point, e.g., tungsten, so that its upper surface is in the same location as will be surface 16 of workpiece 18. The operator then observes the bright spot where the beam 12 contacts the metal piece and adjusts the current in the coil, thereby changing the focal point of the beam, until it appears to the operator as though the bright spot has been minimized.
The visual-manual technique is generally satisfactory at lower current levels, but becomes difficult to apply at higher levels, e.g., currents above 10 mA. It has the limitations or problems in that (1) it is subjective and (2) is only qualitative. It fails to provide a quantitative measure of the beam attributes of focused beam size and of current distribution, i.e., it fails to provide a beam profile.
These beam attributes are influenced by filament current, filament condition, and filament orientation in the electron gun. In comparison to the other components of an electron beam welder, a filament is not durable. A filament's rate of decay varies with the different conditions under which the beam welder is operated. Recommended operating practice is to operate with the filament heated to the "space-charge limited condition" in which a quasi-steady "electron cloud" exists next to the center of the filament. This cloud contains an excess of electrons from which electrons are drawn to form the beam. Overheating the filament shortens its useful life.
To extend filament life, it is desirable to operate at temperatures just high enough to produce a space-charge limited condition. This condition is usually determined by overheating the filament and then observing, as the filament current is decreased, when the beam current begins to decline. When filament current is close to, but less than sufficient for producing the space-charge condition, the beam current does not decrease. Consequently, an operator may select this nearly sufficient current. Beam profile measurements have shown, however, that using nearly sufficient current produces an apparent minimum sized beam but one which is larger and less concentrated than an optimal beam at the surface, thereby reducing weld depth. Measuring the profile of the beam would reveal this deceptive "nearly sufficient" current condition.